The present invention relates generally to a focus detection system used with the so-called phase difference type autofocus (AF) system mounted on single-lens reflex cameras (SLRs) or the like, and an imaging apparatus incorporating it. More particularly, the invention is concerned with a focus detection optical system that, with a taking lens defocused theoretically, enables a defocus direction and a defocus quantity to be so figured out that the taking lens can be more quickly placed in an in-focus state than achieved with the so-called contrast method, and an imaging apparatus incorporating it.
So far, there has been a system comprising a taking lens for projecting a subject onto an imaging plane, a means for splitting or selecting an optical path from the taking lens to the imaging plane (a quick return mirror or half-mirror), a primary imaging plane (predetermined imaging plane) set at a position roughly equivalent to said imaging plane on an optical path passing through them, a field stop located near that primary imaging plane to form a focus detection area, a condenser lens located near the primary imaging plane, aperture stops for implementing pupil division by a plurality of openings, and a plurality of re-imaging lenses and a light receptor element array located corresponding to the respective aperture stops, wherein the condenser lens is adapted to project different areas within the taking lens onto the respective aperture stops, and the re-imaging lenses are adapted to project an aerial image at the primary imaging position onto the light receptor element array through the corresponding aperture stops.
With this system, positions of images projected onto the light receptor element array corresponding to the focus detection area are compared to implement range finding, but constant areas at both ends of the focus detection area are where to obtain defocus information: the focus of the subject at that position cannot be detected.
Known as a typical prior art relying upon such a method is a focus detection system wherein, to implement detection for each of a plurality of focus detection areas, there are a pair of re-imaging lenses provided, and the respective re-imaging lenses are shared upon re-imaging of a plurality of focus detection areas (Patent Publication 1).
As shown in FIG. 20, the prior art comprises a re-imaging lens group 504 comprising a pair of re-imaging lenses corresponding to each of three focus detection areas lined up in one direction on a predetermined imaging plane. The re-imaging lens group 504 is made up of three re-imaging lenses 541, 542 and 543 lined up in one direction; both the outer re-imaging lenses 541 and 543 cooperate to implement focus detection for the center focus detection area, and one outer re-imaging lens 541 and the center re-imaging lens 542 cooperate to implement focus detection for the outer focus detection areas. The prior art also says that the focus detection system may just as well be located in the direction coming out of the paper.
Patent Publication 1
JP (A) 1-266503
However, this focus detection system is designed to implement focus detection using mutually spaced re-imaging lenses for the intermediate focus detection area. Such an optical layout is likely to have an increased angle of incidence to the re-imaging lenses and so be affected by aberrations. An attempt to decrease the angle of incidence to the re-imaging lenses would work against size reductions, because the distance between the focus detection areas and the re-imaging lens group grows long.
Further, the prior art comes up with a plurality of combinations, each comprising at least three re-imaging lenses, wherein a light receptor element array corresponding to the respective re-imaging lenses is independently set up. And focus detection at one range-finding area is implemented by detecting a phase difference in one direction: phase difference information obtained by only one set of re-imaging lenses.
With such an arrangement, an attempt to increase focus detection area size would increase the angle of incidence on each re-imaging lens in the re-imaging lens group, and so tend to produce aberrations. An effort to keep the aberrations in check would bring the re-imaging lenses away from the focus detection area, again working against size reductions.
In view of such problems with the prior art as mentioned above, the invention has for its object to provide a focus detection optical system that comprises a plurality of closely located focus detection areas and is easily capable of making sure focus detection capability while a re-imaging lens group gets close to the focus detection areas, and an imaging apparatus incorporating it. Another object of the invention is to provide a focus detection optical system that has a wide focus detection area and is less affected by aberrations, and an imaging apparatus incorporating it.